Thermal ignition device

ABSTRACT

A thermal ignition device comprising a cylindrical resonating cavity enclosed at one end with a diaphragm and made of a nonconducting material such that, when the air column is stimulated to vibrate by air pressure entering the resonant cavity, an intense heat will be generated at the diaphragm end. A heat sensitive material placed in close proximity to the diaphragm will then be ignited by the heat so generated. This device may be used to detonate a warhead or it may be used to ignite a thermal power supply.

United States atent 1 Fisher Dec. 17, 1974 THERMAL IGNITION DEVICE [75] Inventor: Evan D. Fisher, Chevy Chase, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

22 Filed: Dec. 1, 1967 21 Appl. No.: 687,397

[52] US. Cl 102/70 R, 102/81, lO2/DlG. 5 [51] Int. Cl. F42c 5/00 [58] Field of Search 102/70, 81, DIG. 5, 76

[56] References Cited UNITED STATES PATENTS 2,314,891 3/1943 Moore 102/76 2,487,789 11/1949 Carr et al... 102/70 3,007,412 11/1961 Kipfer 102/81 X 3,277,825 10/1966 Maillard ..l lO2/49.7 3,279,318 10/1966 Vass et a1. 102/76 X 3,362,332 l/l968 Campagnuolo 102/81 Primary Examiner-Samuel W. Engle Attorney, Agent, or FirmSaul Elbaum [5 7 ABSTRACT A thermal ignition device comprising a cylindrical resonating cavity enclosed at one end with a diaphragm and made of a non-conducting material such that, when the air column is stimulated to vibrate by air pressure entering the resonant cavity, an intense heat will be generated at the diaphragm end. A heat sensitive material placed in close proximity to the diaphragm will then be ignited by the heat so generated. This device may be used to detonate a warhead or it may be used to ignite a thermal power supply.

1 Claim, 2 Drawing Figures THERMAL IGNITION DEVICE The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION In the design of fuzes for air-to-air or ground-to-air projectiles the designer is faced with the ever present problem of providing a fuze which will detonate the warhead after a fixed time interval to prevent detonation in possibly friendly areas when the projectiles have missed their intended targets. Previous solutions to the problem have consisted essentially of mechanical or electromechanical devices which have been relatively complicated and are often erratic.

In those fuzes which utilize thermal power supplies it has heretofore been necessary to provide a mechanical striker or an electric match in order to ignite them. As is often the case with the mechanical devices utilized in projectile fuzes, such a mechanical striker might be rendered inoperative or at least erratic by the stresses of the environment in which it must operate.

It is therefore an object of this invention to provide a means by which a heat sensitive material may be ignited without using mechanical or electromechanical devices.

It is another object of this invention to provide a safe reliable fuze which will detonate a warhead that has missed an air target before it falls into possibly friendly areas.

A further object of this invention is to provide a means by which a thermal power supply may be ignited without the use of mechanical or electromechanical devices.

SUMMARY OF THE INVENTION The aforementioned and other objects may be obtained by using a resonant cavity having one end closed with a diaphragm, and a heat sensitive material mounted in close proximity to the diaphragm. A portion of the ram air pressure entering the open end of the cavity is transformed into a substantial amount of heat at the base of the cavity. This heating effect may then be used to ignite the heat sensitive material. In utilizing this device as a means for self-destruction of projectile fuzes the heating effect which occurs after sustained operation for a finite period of time is used to either expand the diaphragm to drive a firing pin into the detonator of the fuze or to trigger or ignite a pyro composition which in turn will drive the firing pin. In using this device to ignite a thermal power supply, match material or other low ignition compositions can be placed on a thin membrane acting as the closed end of the resonant cavity and as the inter-face between the cavity and the thermal power supply. This low ignition material will be ignited by the heating effect on the diaphragm and in turn will actuate the thermal power sup- BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a typical small caliber fuze in which my invention is utilized as a selfdestruction fuze.

FIG. 2 is a cross-sectional view of a fuze having a thermal power supply in which my invention is utilized to ignite the themial power supply.

DESCRIPTION OF THE PREFERRED EMBODIMENTS H. Sprenger in MITTEILUNGEN AUS DEN IN- STITUT FUR AERODYNAMIK Vol. 21, pages 18 thru 34, 1954, (AERE Library Translation 687) disclosed that a resonant cavity, closed at one end, will become heated at the closed end when it is stimulated into vibration by means of a free jet of gas as in the case of a I-Iartmann sound transmitter. The resonant cavity or Hartmann oscillator becomes heated at its base or closed end because of the lack of flow at the base resulting from successive compression cycles associated with the frequency of the device. I then discovered that by utilizing this phenomenon a heat sensitive pyro composition associated with the base end of the resonant cavity could be ignited thereby actuating a fuze.

In FIG. 1 my invention is employed as a self destruction device for projectiles that have missed their intended air targets. In the instant embodiment the device is mounted in the nose of a typical small caliber fuze. This thermal self destruction device comprises a resonant cavity 12 having an open end 11 and a closed end 14; a heat sensitive pyro composition 16 and a firing pin 18. After the projectile is tired ram air enters open end 11 of cavity 12 producing the heating effect, as described by Sprenger, at closed end 14. After a short period of time, the heat transfer through the base 14 of cavity 12 to the pyro mix 16 will initiate the mix forcing firing pin 18 into a detonator 20, mounted in rotor 21, when the rotor has rotated into the in-line or armed position. This will, of course, detonate the warhead thereby destroying the projectile. If the projectile should hit its intended target before sufficient heat'is generated to ignite the pyro mix, the impact of the nose of the projectile against the target will collapse the nose and drive firing pin 18 into detonator 20.

In designing a fuze such as one described in FIG. 1 the problem is reduced to one of simply determining the rate of heat transfer from the base end of the resonant cavity to the expansion device, such as the pyro composition in firing pin in the fuze. Therefore, by selecting materials having an appropriate heat transfer characteristic the fuze can be set to be actuated after a predetermined period of time. An advantage of using a device as herein described is that the temperature in the base of the cavity will rise for an appreciable portion of the flight and will not decay as the velocity of the round decreases.

It is contemplated that in practicing my invention expansion devices other than the pyro mixture discussed herein above may be used. For example, an expanding diaphragm may be used to drive the firing pin into the detonator of the fuze.

In FIG. 2 an embodiment is illustrated in which my invention is utilized to ignite a thermal power supply in a fuze. In this embodiment fuze 30 is equipped with inlet ports 31 which in turn feed plenum 32. Nozzle 34 connects the end of plenum 32 opposite inlet ports 31 to the open end 33 of resonant cavity 36 and it is at this juncture that exhaust ports 37 are also connected. The closed end 42 of resonant cavity 36 is a thin membrane which acts as the inter-face between the base of the resonant cavity and thermal power supply 38. A match composition or other low ignition material'40 may be applied to the power supply side of membrane 42 or to both sides. i I

When the projectile is fired, ram air enters inlet ports 31 and the velocity of this ram air is converted into pressure in chamber 32. Exit nozzle 34 from chamber 32 blows this pressure through open end 33 into resonant cavity 36 thereby creating a substantial amount of heat in and around end 42 of the cavity. Because cavity 36 is constructed of a nonheat conducting material the heat created by the air pressure will not be readily dissipated and will soon ignite match composition 40 thereby actuating thermal power supply 38. The use of a pressure recovery chamber 32 and exit nozzle 34 blowing into cavity 36 permits operation over a large dynamic range of velocity, ballistic match to various firing conditions, and broad band operation.

While the thermal self ignition device of my invention has been described in the context of its application in ordnance projectiles it will be apparent to those skilled in the art that a wide variety of other uses are possible. I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

I claim as my invention:

1. A self ignition device for initiating thermal power supplies, comprising:

a. a hollow cylinder having an open end and a closed end forming a resonant cavity, said closed end being a membrane and said open end being adapted to receive ram air thereby generating a substantial amount-of heat in said closed end;

b. a low ignition composition bonded to said membrane so that said low ignition material will be ignited by the heat generated in said closed end of said cavity;

c. a thermal power supply placed in close proximity to the said closed end of said cavity so that thermal power supply will be initiated upon the ignition of said low ignition composition;

d. a pressure recovery chamber having inlet ports adapted to receive ram air; and

e. a nozzle connecting said pressure recovery chamber to said open end of said cavity. 

1. A self ignition device for initiating thermal power supplies, comprising: a. a hollow cylinder having an open end and a closed end forming a resonant cavity, said closed end being a membrane and said open end being adapted to receive ram air thereby generating a substantial amount of heat in said closed end; b. a low ignition composition bonded to said membrane so that said low ignition material will be ignited by the heat generated in said closed end of said cavity; c. a thermal power supply placed in close proximity to the said closed end of said cavity so that thermal power supply will be initiated upon the ignition of said low ignition composition; d. a pressure recovery chamber having inlet ports adapted to receive ram air; and e. a nozzle connecting said pressure recovery chamber to said open end of said cavity. 